You can arrive before your course starts. This can be either:
You must not travel to the UK before the start date given on your visa, no matter when your course starts.
If you are applying outside of UK, then you should apply before 6 months of course start date. It normally takes 3 weeks to get decision on your application. It can take longer than 3 weeks due to volume in applications.
You cannot travel before the start date mentioned on your visa but you can arrive in UK before your course start date i.e one week if your course is of 6 months or less and one month before if your course is of more than 6 months.
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